Abstract

Background:

Thyroid function depends on the essential trace mineral selenium, which is at the active center of the iodothyronine deiodinase enzymes that catalyze the conversion of the prohormone thyroxine (T4) to the active form of thyroid hormone, triiodothyronine (T3).

Objective:

Because selenium intake in the United Kingdom has fallen during the past 25 y, we wanted to determine whether current selenium status might be limiting conversion of T4 to T3 in the elderly, in whom marginal hypothyroidism is relatively common.

Design:

We investigated the effect of selenium supplementation in a double-blind, placebo-controlled trial in 501 elderly UK volunteers. Similar numbers of men and women from each of 3 age groups, 60-64 y, 65-69 y, and 70-74 y, were randomly allocated to receive 100, 200, or 300 μg Se/d as high-selenium yeast or placebo yeast for 6 mo. As part of the study, plasma selenium, thyroid-stimulating hormone, and total and free T3 and T4 were measured. Data from 368 euthyroid volunteers who provided blood samples at baseline and 6 mo were analyzed.

Results:

Although selenium status at baseline correlated weakly with free T4 (r = −0.19, P < 0.001) and with the ratio of free T3 to free T4 (r = 0.12, P = 0.02), we found no evidence of any effect of selenium supplementation on thyroid function, despite significant increases in plasma selenium. However, baseline plasma selenium in our study (JOURNAL/tajcn/04.02/01720558-200802000-00014/ENTITY_OV0335/v/2017-09-12T073619Z/r/image-png: 91 μg/L) was somewhat higher than in previous supplementation studies in which apparently beneficial effects were seen.

Conclusion:

We found no indication for increasing selenium intake to benefit T4 to T3 conversion in the elderly UK population.